To improve the overall efficiency of a gas turbine engine, a heat exchanger or recuperator can be used to provide heated air for the turbine intake. The heat exchanger operates to transfer heat from the hot exhaust of the turbine engine to the compressed air being drawn into the turbine. As such, the turbine saves fuel it would otherwise expend raising the temperature of the intake air to the combustion temperature.
The heat of the exhaust is transferred by ducting the hot exhaust gases past the cooler intake air. Typically, the exhaust gas and the intake air ducting share multiple common walls, or other structures, which allows the heat to transfer between the two gases (or fluids depending on the specific application). That is, as the exhaust gases pass through the ducts, they heat the common walls, which in turn heat the intake air passing on the other side of the walls. Generally, the greater the surface areas of the common walls, the more heat which will transfer between the exhaust and the intake air.
As shown in the cross-sectional view of FIG. 1, one example of this type of heat exchanger uses a shell 10 to contain and direct the exhaust gases, and a core 20, placed within the shell 10, to contain and direct the intake air. As can be seen, the core 20 is constructed of a stack of thin plates 22 which alternatively channel the inlet air and the exhaust gases through the core 20. That is, the layers 24 of the core 20 alternate between ducting the inlet air and ducting the exhaust gases. In so doing, the ducting keeps the air and exhaust gases from mixing with one another. Generally, to maximize the total heat transfer surface area of the core 20, many closely spaced plates 22 are used to define a multitude of layers 24. Further, each plate 22 is very thin and made of a material with good mechanical and heat conducting properties. Keeping the plates 22 thin assists in the heat transfer between the hot exhaust gases and the colder inlet air.
Typically, during construction of such a heat exchanger, the plates 22 are positioned on top of one another and then compressed to form a stack 26. Since the plates are each separate elements, the compression of the plates 22 ensures that there are always positive compressive forces on the core 20, so that the plates 22 do not separate. The separation of one or more plates 22 can lead to a performance reduction or a failure by an outward buckling of the stack 26. As such, typically the heat exchanger is constructed such that the stack 26 is under a compressive pre-load.
Applying a high pre-load does reduce the potential for separation of the plates 22. However, this approach does have the significant drawback that all the components of the core 20 are placed under a much greater stress than they would be without the pre-loading. In addition, the pre-loading requires that the structure supporting the stack 26 must be much stronger and thus thicker. This pre-load assembly or support structure 40 collectively includes the strongbacks 28, the tie rods 30, as well as the shell 10 structure. This support structure 40 adds to both the weight and the cost of the heat exchanger.
The stack 26 can also be under a further compressive load, which is caused by differential thermal expansion between the core 20 and the support structure 40. As can be seen in FIG. 1, the core 20 is contained in the shell assembly 10. Because the support structure 40 supports the core 20 and is not a heat transfer medium, the components of the support structure 40 are typically made of much thicker materials than that of the core 20. Unfortunately, this greater thickness causes the support structure 40 to thermally expand at a much slower rate than the quick responding core 20 with its thin plates 22. The thickness (and thus the thermal response) of the support structure 40 will also be affected by the amount of the pre-load applied to the stack 26.
Differential thermal expansion between elements of the heat exchanger will cause a compression load to be applied to the quicker expanding sections (e.g. the core 20 and specifically the stack 26). As noted, a compression load is also applied to the stack 26 by the application of a pre-load. Compressive forces from pre-loading and differential thermal expansion can cause a variety of problems, such as fatigue failures, creep and buckling. Buckling is particularly problematic as it results in the slack 26 expanding outward (laterally) in one or more directions. This outward expansion causes the plates 22 to separate from one another, resulting in a nearly complete destruction of the heat exchanger.
An additional source of loading on the heat exchanger can be from the airflow in the core 20. When the inlet air in the core 20 is pressurized, an additional compressive load is applied to the stack 26. This compression loading can also contribute to the occurrence of buckling or other damage. Air pressure loads can further affect plumbing components in the core including the inlet duct 32 and the outlet duct 34. Loads are also created by the pressure of the air in the ducts that carry the air in and out of the core. The duct will carry this load and transfer it to the core 20. Since the core 20 is made of the thin plates 28, to avoid damage to the core 20, only very limited loads can be applied to the core 20.
In addition, the core 20 can also experience loads caused by external forces. Such forces include inertia loads, which occur in mobile applications, and loads transferred through the ducts from the attached plumbing, such as those caused by turbine vibrations. Inertia loads can be created by accelerations (such as changes in direction or speed) applied to a vehicle in which the heat exchanger is mounted. For example, a vehicle traveling over uneven terrain can cause various inertia loads to be applied to the heat exchanger. Inertia loads increase the likelihood of buckling by providing forces in a variety of directions including those which are aligned with, and perpendicular to, the compressive loads. The aligned inertia loads increase the potential for failure by being additive to the compressive loads. Whereas, the inertia loads directed perpendicular to the compressive loads, increase the likelihood of failure by encouraging the core to buckle to one side or the other. Similarly, the forces that are transmitted through the ducts have the potential to cause failures in the thin plates 20 at locations where the ducts contact the thin plates 28.
As shown in FIG. 2, prior approaches to minimizing differential thermal expansion loads on only the core 20, have included the use of a bellows 36. Bellows function by expanding or contracting to accommodate the relative thermal growth.
Unfortunately, bellows typically have notable drawbacks, including that they are expensive, difficult to assemble and add additional leak paths to the heat exchanger. Such leaks greatly reduce the efficiency of the heat exchanger. Bellows also must be repaired or replaced frequently.
Therefore, a need exists for a heat exchanger that provides sufficient column stiffness for the core structure to prevent buckling and which can carry loads created is by the air pressure within the core. The heat exchanger's increased core column stiffness should significantly reduce the amount of pre-load applied to the core. This in turn will result in reduced structure needed to contain the core, as well as, reduced differential thermal expansion between the core and the shell. The heat exchanger should further be able to accommodate differential thermal growth without the use of a bellows system or other type of variable position linear force system. A heat exchanger with such increased column stiffness will enable the heat exchanger to withstand higher inertia loads. A need further exists for a heat exchanger that can distribute the loads from the ducting into the core structure without causing damage to, or a failure of, the core.